The Japanese have continuously shrugged off responsibility for their past military actions when they took control of other Asian countries (most notably in China and Korea), or choosing to deny/change facts (i.g. comfort women).
By Miyavi accepting this role, the Japanese felt that he “betrayed” his country with his portrayal, despite his comments that he portrayed this role so that it would be respectful to the Japanese, but honest in the actions. This, however, is just more of the population’s revisionist thinking when it comes to the war, spearheaded by Prime Minister Shinzo Abe.
Despite this, Miyavi has repeatedly said in interviews that he hopes the movie gets a premiere in Japan or that it gets seen by the people, so that, yes, they see what transpired during the war, but that they also see the other big message in the film, that of acknowledging your actions and forgiving.
That is why there’s comments now using his ethnicity against him and demanding he be deported (despite the fact that his mother is ethnic Japanese and his father is Zainichi Korean), even though Miyavi has always been proud of his heritage.